Passive House Ventilation and Heat Recovery: ERV/HRV/MVHR by Zehnder
|Zehnder Novus 300 (aka Paul) ERV/HRV|
Passive houses by design are extremely airtight enclosures (often an order of magnitude tighter, or more, than a conventionally constructed building). So much so, they require mechanical ventilation to provide fresh air.
An air-tight building that was not mechanically ventilated would quickly result in a toxic place. The expended air of occupants, elevated levels of humidity that would result (leading to mold formation), and accumulation of any off-gassing of VOC-based products would quickly lead to health-hazards. Mechanical ventilation of air-tight homes is absolutely essential.
Fortunately, an MVHR (mechanical ventilation heat-recovery) exists to provide passive house ventilation of fresh air while removing stale air in a manner that retains most of the heat or coolness of the interior living space. Passive house ventilation employing ERVs provides occupants with an extremely high quality of indoor air.
During the early planning and PHPP modeling stages of our passive house, we conservatively assumed that our ACH50 air-tightness rating was going to be at the Passivhaus minimum requirement of 0.60, although we fully intended to substantially exceed that number during construction.
Given the architectural layout and the amount of glazings we chose (especially on the north side of the building) we wanted to maximize the heat exchange efficiency of the mechanical ventilation system we would use as much as we could.
After consideration of several vendors, we selected two MVHRs from Zehnder for our passive house ventilation.
Other manufacturers* we considered (primarily UltimateAir) offered units with somewhat lower rated efficiencies (83% at the time of our construction), but we couldn't leave anything to chance. As I indicated in an earlier article, since we consciously made some design decisions that "bent the rules," we needed to extract as much efficiency in every component that we could to make up for those decisions and that certainly included our passive house ventilation systems.
A key driver that led us to this decision were their extremely high Passivhaus certified heat recovery efficiencies of 92% and 93%.** Additionally, when the units are configured as ERVs (energy recovery ventilators)--they can be configured to operate in either mode--65% of the moisture contained in the extract air is transferred from the exhaust air to the supply air. MHVRs configured as HRVs exhibit slightly higher efficiencies than ERVs. We chose the latter ERV configuration as it was more appropriate for our climate.
|Zehnder performance claims are certified by PHI as being valid|
To satisfy the ASHRAE nominal air exchange rate of ~0.3 ACHN (air changes per hour) we needed a system capable of delivering about 300CFM during normal operation--appropriate ventilation for approximately 6600SF TFA (treated floor area) or conditioned space. The total cubic volume of the home was estimated to be about 60,000 cubic feet.
We discussed our design objectives with Barry Stephens of Zehnder America and he configured a two unit solution, based upon our architectural plans, consisting of a Zehnder Novus 300 (Paul) and a Zehnder ComfoAir 200. In fact, I believe, our project was the first in North American to actually use the Paul unit.
Interestingly enough, while the Zehnder Novus 300 or Paul ERV has more sophisticated (newer) electronic controls, the Paul unit requires hardwired connections for remote control of its ventilation rates (only high/normal).
The ComfoAir 200 HRV ventilation rates are controlled by wireless switches that can be easily mounted anywhere an exhaust/return register is located (namely areas of potential high humidity, such as bathrooms and laundry rooms) and those controls allow for more precise control of air-flow rates.
|Zehnder ComfoAir 200 HRV|
When we first received our units, there was a small kink that needed to be ironed out. The installation manual for our Paul unit was in Swiss! That was fun, to be sure. Of course, with the expert guidance of Barry, the units went in without a hitch.
I must take a moment to acknowledge how wonderful of an experience it was to work with Barry Stephens. He made several long-distance trips to our home (from New Hampshire) to assist in the actual layout, installation, and commissioning of both systems. Our HVAC sub-contractor had not had any experience installing Zehnders, but Barry was able to instruct him in-person. Barry's knowledge and commitment to Zehnder and their clients are most noteworthy.
Making a leap of faith, we decided to locate our space heating and air-conditioning equipment, ductless mini-splits at strategic locations in the home and rely on the Zehnder ventilation system for effective distribution and destratification of the conditioned air throughout the whole house (more on this later)--Barry told us that this method was being used abroad with a good measure of success.
One of the especially nice capabilities of these ERVs is that they can easily be converted into HRVs (heat recovery ventilators) by simply swapping out the enthalpy heat exchanging unit with one that does not provide for moisture recovery between the intake (fresh air) and the exhaust (stale air).
From an energy consumption stand-point, for the amount of work they perform, these units consume very little energy. Here's what we measured (at 240v):
|Ventilation Rate||Both Units||Novus 300 (Paul)||
|Level 3 (Highest rate)||0.7A||< 0.4A||< 0.4A|
|Level 2 (Normal rate)||0.4A||0.2A||0.2A|
|Level 1 (Low rate)||0.2A||0.1A||0.1A|
|Level (Unoccupied/Absent)||0.1A||< 0.1A||< 0.1A|
To enhance ventilation efficiency, we connected our earth air tubes to the intakes of both units. This allows us to significantly reduce the temperature delta between the incoming fresh air and the outgoing stale air. Absolute efficiencies do, however, decline somewhat as the intake and exhaust temperatures get closer to one another. Another benefit is during the summer months our earth tubes cool and dehumidify the incoming air before it makes it to the intakes of the ERVs.
Both Zehnders can be configured to operate in by-pass mode which allows the fresh intake air to be directly fed into the home without passing through their heat-exchanging function. This is especially convenient for us, since we are using earth air tubes to pre-cool (to 69-70F even when its 100F outside) and pre-dehumidify air.
Bypass mode eliminates the potential of unnecessarily transferring additional heat from the exhaust air to the incoming air during the cooling season so we can maximize the free air-conditioning we receive from our earth tubes. This bypass feature is user programmable to engage at desired set-temperatures.
Beyond merely providing fresh air to the building, the ERV/HRV/MVHR units "vacuum" the air with the use of filters with a MERV (Minimum Efficiency Reporting Value) rating of 7/8 to as high as 13. Filters with MERV ratings of 13 can perform practically as effective as a HEPA (High-Efficiency Particulate Air) rated filters (MERV 16). Filtering eliminates most impurities, including pollen and mold-spores, as well as removing indoor air-pollutants such as dust. Supply and extract air flows are kept completely separate preventing transfer of odors or bacteria which further improves interior air freshness.
Having lived with the Zehnders now for more than a year, I must state that these systems and the company that manufacturers them are thoroughly thought out, evolved, and refined.
Using mechanical ventilation in a passive house results in the highest levels of indoor air-quality that can be achieved.
Subsequent to the completion of our home, UltimateAir is now advertising their RecoupAerator 200DX units as being 95% efficient. I am not sure if they have or are seeking PHI certification of that claim.
With respect to MVHR efficiencies, be mindful of stated operating efficiencies such as Apparent Sensible Effectiveness (ASE) numbers. While some MHVRs may boast highly efficient numbers, they may be misleading and/or inflated. The reason being there can be cross flow leakage within the heat exchanger unit itself, which would tend to increase supply temperatures and falsely inflate heat recovery efficiency numbers.
The standardized test of CSA C439M for MVHR heat recovery efficiency ratings includes the effect of motor heat and cross leakage gains. It is usually numerically higher than the sensible recovery efficiency of the MVHR. The Home Ventilating Institute (HVI) certifies units utilizing this testing methodology. The Passivhaus Institut requires the cross leakage rate to be no more than 3%. Zehnder easily exceeds that stringent requirement.
Before making a selection of an MVHR, we would suggest you confirm certifications from both PHI and HVI. Like air leakage rates, too low performing MHVRs can have a dramatically negative effect our overall real-world performance of a Passivhaus building. In other words, do your homework.